Low impedance oscillator



Nov, 26, 1946. T. L. WILSON LOW IMPEDANCE OSCILLATOR Filed July 20, 1942 INVENTOR v; Fla/Ms [Ana/v7 Mum ATIORNEY ployed.

Patented Nov. 26, 1946 Thomas Lamont Wilson,

signor to Federal Telegraph Company, a corporation of California Scotch Plains, N. J., as-

Newark,

Application July 20, 1942, Serial No. 451,596

3 Claims.

This invention, relatesto an electronic oscillator, the output impedance of whichis relatively low. I

In many industrial fields, it is desirable to couple the output of an electronic oscillator to a utilization circuit or load having a very low impedance relative to the output impedance of the conventional oscillator and to have the voltage across this load impedance remain approximately constant when the magnitude of the impedance changes over a wide range. The conventional electronic oscillator, having a relative ly high internal impedance will not provide a constant voltage source for a load impedance whose magnitude varies widely.

One object of my invention is to provide an electronic oscillator capable of feeding a load circuit having an impedance whose magnitude is-relatively low and varying, and of keeping a substantially constant voltage across that impedance.

In electronic oscillators as used in the prior art, the output has customarily been taken from an impedance located directly in the anode circuit of an electronic tube, the excitation of the tube being accomplished by feeding energy, in the proper phase relationship, to an input circuit constituted by the control grid and cathode of such tube. With electronic tubes of types which are available in the art, the output impedance of a tube connected in the manner just described is relatively high and therefore the external impedance which feeds the load, must for the sake of efficiency and voltage regulation be made. quite high relative to the anode-cathode internal impedance of the tube, which latter value is high, relative to values prevailing in generators of the non-electronic type.

In carrying out my invention, I make use of electronic tubes of conventional-types, having a comparatively great anode-cathode impedance, but I so connect the input and output circuits, respectively, of such tubes that the impedance of the external output circuit, to which the load is connected, shall be efiiectively only a fractional part of the impedance existing when conventional methods of connection are em- Another purpose of this invention is to provide a generator of electronic oscillations having an external output circuit of such low impedance as to possess an inherent quality of good regulation, when connected to a load of relatively low impedance.

Referring now to the drawing,

there are shown two electron tubes, Ill and II, coupled to one another so as to constitute an electronic oscil-j lator. While these tubes have for the purpose of simplicity in illustration been shown as triodes, it is to be understood that my invention is applicable to tubes having a larger number of elements, such as tetrodes or pentodes and the modifications of my invention necessary to adapt it for use with such last mentioned types of tubes, will be apparent to those skilled in the art.

Thecathodes l2 and 13 of tubes [0 and II, respectively, may be heated from any convenient source, as conventionally shown in the drawing by the symbols LT applied to the v leads proceeding from such cathodes. Likewise the cathode of tube It is preferably grounded with respect to oscillatory currents by means of bypass condensers Hl connected in series across the extremities of the cathode and having a direct ground lead extending from the conductor between the two condensers, as indicated at l5. At 56 is shown the control grid, and at I! the anode of tube It. The exciting energy supplied to grid I6 is derived in a manner which will be herein later described, but for the purposes of the present explanation, it is to be assumed that oscillatory electrical energy reaches grid [6. Anode I1 is supplied with suitable high tension energy derived through conductor 18, as indicated by the legend H'I+. This energy reaches the anode through a suitable impedance such as inductance l9, connected in series therewith. The amplified oscillations appearing in the circuit of anode I! are transferred through the combined action of impedance [9 and coupling condenser 20, to the grid 2| of tube ll.

This last mentioned grid is, with respect to oscillatory energy, isolated from the ground by means of a suitable impedance such as inductance 22 and resistance 23, connected in series in the ground circuit of the grid. Likewise the static potential of grid l6is determined by its connection to the ground through a suitable impedance such as inductance 24 and resistance 25, while the source of high tension supplying energy to the conductor I8 has its negative terminal connected to the ground, so that condenser 26 serves to by-pass oscillatory energy around this source.

' The anode 21 of tube II is connected directly. to a suitable source of high tension energy as indicated at 28, which generally, but not necessarily is the same source as for anode ll of tube ill. This source likewise has itsnegative ter-,

minal grounded as indicated at 29, while condenser 30 serves to by-pass oscillatory energy around this last mentioned source. The oscillatory energy reaches cathode l3 of tube II through condensers 31. Between the mid-connection point of condensers 3| and the ground, is interposed a tank circuit comprising a condenser 32, preferably variable, and aninductance 33 connected in shunt therewith. The constants of this tank circuit serve to determine the frequency of the oscillations produced by my generator; and such oscillations may lie either in the audio frequency or in the radio frequency spectrum, according to the choice of such constants. The exact point of grounding this tank circuit may conveniently be determined by adjustment of the point upon inductance 33 to which ground lead 35 is connected. The upper end 35 of the tank circuit-is connected tocathode l3 via condensers 3i, while the lower end 36 of this circuit is coupled back through condenser 31 to grid l6 of tube Ill. The tank circuit 32, 33 constitutes the output circuit of my generator and it may conveniently be coupled to the load by means of an inductance, 38 placed in juxtaposition to inductance 33 although other methods of coupling to the load, as well known in the art, may be employed.

The way in which oscillatory energy reaches the tube H is such as to cause the efiective input circuit thereof to lie between the grid and the anode, rather than between the grid and the cathode, since the input voltage appears across points 39 and 4B, the latter being at ground potential, while anode 21 is also at ground potential with respect to oscillatory energy, due to the action of by-pass condenser 30. The load circuit thus becomes one of the type known as cathode follower, in that the load circuit is connected so that not only the output energy in reaching the cathode passes through such load circuit, but also the return between the grid and cathode takes place through this same load circuit.

In the operation of my invention, tube I0 functions chiefly to amplify the oscillatory energy impressed upon the grid IS, the amplified oscillations being passed on to grid 2| of tube II by means of the impedance coupling circuit including impedances l9 and 22 and coupling capacity 20. The action of this tube and its coupling circuit will be apparent to those skilled in the art. Tube H has the input energy impressed between grid 2i and anode 21, while the output thereof is obtained between cathode l3 and a point of inductance 33 which is at ground potential. When a tube is connected in this fashion, its in- 4 ance l9 to increase, thu depressing the potential reaching grid 2|.

The total voltage between grid 2| and cathode I3 will be the grid-ground voltage less the cathode-ground voltage. By choice of suitable values of the various impedances and capacities, as well known in the art, phase shifts efiects may be kept at a minimum.

A reduction of voltage on grid 2| will lower the voltage drop across the tank circuit and thus will increase the net or effective anode-cathode voltage. This brings about the result that the voltage appearing from cathode to ground is in phase with the grid-ground voltage. The voltage appearing at point 36 will then be 180 out of phase with the grid voltage and thus, when fed through coupling condenser 31, will appear upon grid l6 of tube H1 in proper phase to sustain oscillation of the system.

Since the efiective grid-cathode voltage of tube l l depends on the relative grid-cathode and cathode-ground impedances, if a load be coupled into inductance 33, the cathode-ground impedance will be lowered and the net grid-cathode voltage will therefore be raised, assuming that the driver voltage supplied to tube l I remains constant. While this elfect of lowering the cathode-ground impedance will usually be less at low frequencies than at high frequencies, yet the effect is present under all conditions of oscillation. The cathodeground voltage supplied by tube II depends upon 4 the oscillatory current in the anode circuit and ternal output resistance, as ordinarily measured between anode and cathode, will be multiplied by a fraction, the value of which depends upon the amplification constant of the tube. Thus even a tube having a very high IL may be employed as tube I, notwithstanding the fact that such tubes, as usually connected, have an extremely high output impedance.

Tracing the path of one cycle of oscillation through the circuits of my invention, let it be assumed that grid I6 is driven positive above its static potential. This will increase the anode current of tube In and since the internal voltage drop of the tube plus that of the voltage drop across impedance l9 will always add'together to equal the voltage supplied by the high tension source through conductor l8, this increase 'of anode current will'cause the voltage across impedupon the cathode-ground impedance of this tube.

If this output impedance be lowered, an increase in oscillatory current in the anode circuit, resulting therefrom, will tend to keep the cathodeground voltage constant. Thus it will be seen that there results a regulatory action similar to that obtained in the case of a generator having low internal impedance. 3

Various modifications of this invention may be made in order to adapt it to special uses. For example tube 10 may be excited from an independent source, rather than receiving its excitation energy through feed-back condenser 31. Optionally, it would be possible to substitute for tube II a pair of tubes connected in push-pull fashion. Other modifications of my invention will be apparent to those skilledin the art.

' WhatI claim is:

1. A low impedance oscillator including two electronic tubes, having cathodes, control grids,

and anodes, means for supplying suitable energy to the respective cathodes and anodes, means for mean potential, means for coupling the output anode-cathode circuit of the first tube to the input grid-anode circuit of the second tube, circuit means connecting the oscillatory energy of the anode of the second tube to ground, output circuitmeans connected in the common grid-cathode and anode-cathode return circuit of the second tube, means for withdrawing oscillatory energy from said output circuit means, and means for feeding back to the control grid of the first tube, in regenerative phase relationship, a portion of' the energy present in said output circuit means.

2. A low impedance oscillator including a first electron discharge tube having a grid-cathode input and an anode-cathode output, a second electron discharge tube having a grid-anode in put and a cathode-ground output, means coupling the output of the first tube to the input of of anode energy, capacitive means by-passing said source, a coupling impedance in the anodecathode circuit, a second tube having cathode, grid and anode, a choke coil and resistance in series between the grid andground to bias said grid, cathode-heating and by-passing means, a

source of;anode energy, passing said source directly to ground, a resonant tank circuit connected between cathode and ground including a series inductance and means for grounding a predetermined point of said inductance, means for withdrawing energy from said tank circuit, means for feeding back a portion of said tank circuit energy to the grid of said first tube, and a capacity coupling the anode of said first tube and the grid of said second tube,

whereby said fedback energy is in regenerative phase relationship and whereby said first tube excites said second tube so as to produce a selfoscillatory system.

THOMAS LAMONT WILSON.

capacitative means by- 

